Sharp says Cotter broke the tribe's trust and violated its sovereignty by pursuing a case against Shannon Augare, who also serves as a state senator.
Augare has been charged with drunken driving, obstructing a peace officer and reckless driving.

"We want to make it very clear that your office took jurisdiction in this case without any consultation with the government body and government of the Blackfeet Tribe and against the laws of the Blackfeet Tribe," Sharp wrote in a letter to Cotter, the Associated Press reported. "We are very disappointed with your action and believe you have breached the trust relationship between your office and the Blackfeet Tribe."

The incident occurred on the reservation. But Augare says federal prosecutors lack jurisdiction over him.

The tribe's prosecutor looked into the incident but never filed charges in tribal court.